The present invention relates to imaging sensors utilized in conjunction with a vehicle wheel alignment system to acquire data with are either representative of vehicle wheel alignment angles, or from which vehicle wheel alignment angles can be calculated, and in particular, to an improved machine vision vehicle wheel alignment sensor incorporating a scanned beam imaging system to generate an image associated with a vehicle wheel from which alignment angle information can be determined.
Aligning vehicle wheels within specific tolerances is important for optimal control of the vehicle and for consistent wear of the tires. Alignment is performed primarily by adjusting camber, caster, toe, and steering axis inclination. As part of calculating the alignment angles for the vehicle, the angles of the wheels must be determined. The angles can be determined relative to an external reference, such as found in machine vision systems, or relative to the other wheels, such as found in wheel-mounted systems. It is known that these angles can be measured using an electro-optical transducer that incorporates a solid state detector array. In the case of machine vision systems, the detector array may have multiple columns and rows forming an area to capture a two-dimensional image, and in the case of wheel-mounted systems, the detector array may only need to be linear, having a single row with as few as two receptor elements. In either case, the image on the detector must be analyzed meticulously so that accurate alignment angles can be calculated.
Wheel-mounted alignment systems typically have sensor heads on each wheel of the vehicle, and each sensor head has an emitter and a receiver that works in combination with at least one other sensor head along the vehicle's sides and across the vehicle. The receiver units may have photodiodes as set forth in U.S. Pat. No. 4,302,104 or a charge coupled device (CCD) as set forth in U.S. Pat. Nos. 5,018,853 and 5,519,489, and the emitter units may have a single source as in U.S. Pat. Nos. 4,302,104 and 5,018,853 or multiple sources as in U.S. Pat. No. 5,488,471. Angles and distances are calculated according to the positions of the spots or lines that are detected by the linear arrays.
Machine vision vehicle wheel alignment systems typically use a solid state camera with an array detector mounted away from the vehicle to obtain an image of a wheel mounted target. The target incorporates an accurately reproduced pattern that has known control features, as set forth in U.S. Pat. No. 6,064,750. The position of the features in the image are found and the orientation of the wheel can be calculated by well known algorithms. Some machine vision systems do not use a predefined target but identify either random or predetermined geometric features directly on the wheel or tire of a wheel assembly, such as projected light stripes or the circular wheel rim, and use the distortion of the geometry to determine positions and orientations.
In wheel alignment systems, the imaging requirements of the optical sensors are somewhat different than those of a standard camera. Very precise measurements must be made at a rate of at least 2 Hz. on static or very nearly static scenes. This requires stable low-noise images. The accuracy of the measurement depends on the precision with which image features such as edges, centroids, corners, lines or boundaries can be determined. Methods for analyzing the image obtained using a standard area imaging sensor must take into account the possible sources of inaccuracy and compensate for them.
Standard area imaging sensors suffer from a number of restrictions when utilized in an automotive service environment. A typical standard area imaging sensor utilizes a fixed focal length lens assembly to acquire images of a field of view which is sufficiently large enough to accommodate the varied positions of vehicle wheels associated with different types of vehicles. The region of interest which represents a vehicle wheel or alignment target within the field of view generally corresponds to only a small portion of the acquired image, and accordingly, comprises less than the maximum possible number of image pixels. Additionally, standard area imaging sensors are susceptible to specular reflection problems, such as may arise when bright lights are reflected from shiny or chromed surfaces of a vehicle or vehicle wheel rim.
In contrast to a standard area imaging sensor or camera, a scanned beam imaging system or laser camera can selectively image a region of interest within a field of view, provides adjustable image resolution for the region of interest, and is less susceptible to specular reflection problems. A scanned beam imaging system obtains an image by scanning a laser spot or other form of illumination across area region of interest within a field of view, and detecting the light reflected back to the imaging system from the illuminated surfaces. The image generated by a scanned beam camera is built up by associating the measured properties of the reflected light with the direction of the incident illumination at many points over time. For example, if a laser spot is swept over an area in a raster pattern and the intensity of the reflected light is recorded at discrete points in the raster, a two-dimensional image similar to that acquired by a standard camera can be built up from the individual points or pixels.
Scanned beam cameras have a number of advantages over conventional area imagers used in alignment systems. For example, a scanned beam camera is well suited for close-up imaging. The scanned beam camera combines bright target illumination with a small aperture to obtain a large depth of focus. While the “shutter speed” for a single frame acquired by a scanned beam camera may be the same as that of a conventional area imaging camera, for individual pixels the scanned beam camera is thousands of times faster. In effect, the scanned beam camera is acquiring millions of individual pictures each second, so motion blur is virtually eliminated.
With a scanned beam camera, there is no image distortion caused by light passing through glass lenses. The illuminating light or laser beam is generally steered with a mirror to define the location of the pixels in the acquired image. Reflected light is generally collected by a detector with a lens on it, but this lens is used only to concentrate light from an area onto a single high speed light sensor or receptor. Light wavelength and rate of signal intensity change are used to identify the reflected light, not a location on a two-dimensional imaging array.
The image resolution of a scanned beam camera can be varied by changing the frequency at which the reflected light is sampled, and by changing the distance between adjacent scan lines illuminating a region of interest within a field of view. This can allow for fast searches of a large field of view at a low image resolution, to identify the location of a desired target or region of interest in the field of view, and facilitates accurate tracking of the desired target with a higher resolution in a selected region of interest once the target has been located. This is similar to decimation and sub-windowing on CMOS-type area image sensors currently used in existing machine-vision vehicle wheel alignment systems, but has fewer limits on the allowable resolutions. For example, this feature of a scanned beam camera bypasses one of the basic limits of conventional array imagers, i.e., the number of pixels physically disposed on the imager sensor.
Scanned-beam cameras also offer another unique feature called confocal imaging. The basic principle of confocal imaging is that the scanned illumination beam can be manipulated in three dimensions (x, y, and z axis) and filtered back through a tiny aperture so that three-dimensional objects under very high magnification remain in sharp focus because they are not blurred by the reflected light bouncing back to the receiver at different angles from different depths. Confocal imaging has found application in specialized medical, scientific, or industrial imaging where a combination of high magnification and high resolution are desired, but has not been adopted for use in vehicle service applications.
Machine vision wheel alignment system designs currently trade off between the size of the field of view and image accuracy. Even with the trend to incorporate more pixels on conventional image sensors, this trade off makes some aspects of machine vision wheel alignment system designs impractical. Scanned beam camera “zooming” or viewing of a selected region of interest within the field of view at a high resolution can go beyond this limit by allowing a large or full field of view to be used for target searches, then “zooming” or selectively viewing an identified region of interest or target utilizing substantially the entire photosensitive surface of the imager to obtain a locally high image resolution. The effect is equivalent to providing an image sensor with a very large number of pixel elements, most of which are not utilized when viewing a limited area or region of interest
One difference between conventional image sensors and scanned beam cameras is the pixel configurations. On a conventional image sensor, each pixel is configured as a roughly rectangular sensor element that integrates the received light. In the scanned beam camera, the illuminated region configuration determines the effective pixel configuration. To take maximum advantage of the variable zoom and resolution, the illuminated region size can, and should, be adjusted so that it is comparable to the spacing between adjacent illumination raster scan lines of an image.
The depth of field in a scanned beam camera is superior to that of a conventional image sensor because of the removal of most focusing optics. With conventional image sensors, the focusing optics impose physical design limits which restricts possible configurations of vision-based aligners.
In contrast to conventional image sensors, undesired specular reflections are reduced, because the scanned beam camera ignores light from sources other than the associated illuminating light source, such as a laser.
Similarly, ambient light problems commonly found in images acquired by conventional image sensors are reduced when a scanned beam or scanned beam camera is utilized, because the scanned beam camera ignores slowly changing light sources, like sunlight, and because the scanned beam camera ignores light from sources other than the illuminating light source, such as a laser.
There are a number of commercial systems currently available which utilize a scanned beam camera technique to gather image data. These systems generally employ rotating mirrors to steer the illuminating light or laser beam, and are not well suited for use in automotive aligner applications because they are relatively expensive, delicate, and require significant maintenance. Micro-electromechanical (MEMS) mirror based scanned beam camera designs which replace the rotating mirrors with controlled arrays of tilting mirrors, have recently been announced. While not currently utilized in any automotive service applications, a MEMS mirror based scanned beam camera could potentially reduce the cost, and increases the reliability, of a scanned beam camera system, and render such systems practical for use in an automotive alignment system.
During operation, the MEMS mirror is selectively controlled to direct an illuminating light source to sequentially illuminate each pixel in a field of view. Light reflected from the illuminated pixel is gathered, often through a large numerical aperture, and converted to electrical signals using a photodetector. Beam angle sensor technology may be used to correlate the sequence of reflectance values to specific pixel locations, thus creating a digital image of the field of view. Images collected may be monochromatic, color, and even infrared, depending on light sources and detectors used. The image produced by a MEMS-based scanned beam camera is a two-dimensional raster image similar to that produced by an imager chip in most aspects. Compared to alternative imaging technologies, MEMS mirror based scanned beam cameras offer greater depth of field, reduced motion blur, enhanced resolution, extended spectral response, reduced cost, reduced size, lower power consumption, and improved shock and vibration tolerance.
Alternative configurations of scanned beam cameras measure the time-of-flight property of the light from the light source or laser emitter to the scanned beam camera sensor, and calculate associated distance information for each discrete image pixel. These systems are commonly utilized for reverse engineering sculptured parts or capturing artwork, and utilize a rotating mirror system to produce raster patterns or spiral patterns. These systems could be modified to utilize a MEMS mirror system in place of the rotating mirror system.
The illuminating light source or laser beam used by a scanned beam camera system can be a combination of a number of different illumination sources, each having a different color. Combining red, blue, and green light sources or lasers results in a visible color image comparable to that produced by a conventional color imager sensor. Light frequencies outside the normal RGB space used in commercial cameras can also be used, allowing a larger color space to be produced that has advantages in identifying specific surfaces, such as rubber. This approach also allows the use of 4 or more beam colors, which has a similar advantage in identifying surfaces that may selectively reflect narrow color bands, such as rubber or road dust.
In addition to the aforementioned advantages of scanned beam cameras, a multi-color scanned beam camera system shares a key characteristic with a Foveon color imager sensor: the three or more colors levels associated with each pixel in an image are measured at the same point in the image. This allows higher resolution, and allows the many advantages of a high resolution color camera to be attained. Light from multiple light sources can be detected in parallel by multiple detectors, each with their own bandpass filter.
Although scanning scanned beam camera systems have not been adapted for use in the generally harsh vehicle service environment, it would be advantageous to provide a vehicle wheel alignment system with an imaging sensor utilizing a scanned beam camera or imaging system in place of a conventional area imaging camera or sensor to utilize the advantages in resolution and flexibility offered by a scanned beam camera system.